Countercurrent flow counters

The best known use of the hairpin is its operation in true counter-current flow which yields the most efficient design for processes that have a close temperature approach or temperature cross. However, maintaining countercurrent flow in a tubular heat exchanger usually implies one tube pass for each shell pass. As recently as 30 years ago, the lack of inexpensive, multiple-tube pass capability often diluted me advantages gained from countercurrent flow. 

Figure 8.8. Plate and spiral compact exchangers, (a) Plate heat exchanger with corrugated plates, gaskets, frame, and corner portals to control flow paths, (b) Flow patterns in plate exchangers, (i) parallel-counter flows (ii) countercurrent flows (iii) parallel flows throughout, (c) Spiral exchanger, vertical, and horizontal cross sections.

The effect of gas has been considered by Takriff and various colleagues. With countercurrent flow of liquid and gas the column can flood. Flooding occurs at a constant liquid flow rate when the gas flow rate is increased to the point where all the liquid entering the top of the column cannot flow down the column counter to the upward flow of gas through the stage divider openings. The column is flooded when the excess liquid entering the column must flow out the top of the column with the gas. 

Flow arrangements for hollow-fiber membranes (a) countercurrent flow (ft) parallel and counter-current flow (c) radial crossflow. 

Because there are two flowing phases, the possibility exists to feed them either concurrently in up- or downflow, or counter-current ly. Whether a concurrent or a countercurrent flow is used depends on throughput, heat recovery and availability of driving forces for mass transfer and chemical reaction. The phenomenon of flooding, known from countercurrent flow, does not appear in concurrent flow, thus allowing for much higher flow rates. 

A hollow fibre module can be operated in three different modes co-current, cross, or counter-current flow. In co-current flow, the permeate flows in the same direction as the feed and retentate. In cross-flow, the permeate flows perpendicularly to the feed and retentate while in countercurrent flow the permeate flows in the opposite direction. The countercurrent flow pattern gives the best performance as the driving force for transport is maximized along the module length. One can produce an arbitrarily high purity... 

Continuous fractionation is carried out in countercurrent flow. For the continuous fractionation of a solution containing two dissolved substances, a split column is used (Fig. 7-30). In the upper column section, the crystallization column, the less soluble component crystallizes during the counter-current contact of the mother liquor and the crystal. In the lower section, the concentrating or enrichment column, the more soluble component is favorably extracted from... 

Countercurrent multiple contact. This method involves the use of a cascade of stages, extracting solvent and solution to be extracted entering at opposite ends of the cascade. Extracts and raffinates flow counter-currently. The operation is more analogous to gas absorption than to any distillation practice. It is necessarily continuous but may be simulated in batch fashion, in the laboratory for example, as pseudo countercurrent multiple contact. ... 

The countercurrent and cocurrent flow models may also be approximated by a series of perfect mixing blocks as described for the cross flow model. The countercurrent flow model would require an additional iterative loop to converge the recycle created by the counter flowing permeate stream. 

Fixed Tubesheet—The advantage of a fixed tubesheet exchanger is that it is a true countercurrent flow exchanger and no reduction of the log mean temperature difference (LMTD) is required for non-true counter-current flow between the shell-side fluid and the tube-side fluid. However, it s impossible to mechanically clean the shell side, as the bundle cannot be extracted from the shell. When such an exchanger fouls on the tube side, it can easily be cleaned. When it fouls on the shell side, you can throw the entire exchanger away. 

The sketch in Fig. 16.4 indicates clockwise flow in the hot primary loop and the cool secondary loop. This arrangement indicates countercurrent flow in the BCs and coupling HEXs. Note that there are four equally likely arrangements of the flow in the loops for the system shown in the figure clockwise or counter-clockwise in either loop. 

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